Scrub dangling Janet references; drop dead Janet-coupled files

Rephrase comments that pointed at deleted Janet files (emit.janet, the seed
sources, 'the Janet back end punts ...') to present-tense descriptions of the
Chez behavior. Comment/docstring-only; the self-host fixpoint is unchanged
(comments don't affect the compiled seed).

Delete five files that were Janet-host shims with no Chez path: clojure.java.io
(provided natively by host/chez/io.ss), and jolt.{nrepl,png,interop,shell}
(the janet.* bridge, os/shell, janet.net — none exist on Chez).

jolt-cf1q.6

host/chez/natives-array.ss

@@ -3,8 +3,7 @@
 ;; A jolt-array wraps a Chez mutable vector + a `kind` tag (for bytes?). The array
 ;; CONSTRUCTORS are native (they build the backing); the overlay's aget/aset/alength
 ;; are pure over count / nth / jolt.host/ref-put!, so we extend those dispatchers
-;; to see a jolt-array. Mirrors src/jolt/core_refs.janet (Janet uses its mutable
-;; arrays/buffers; here a Chez vector). Loaded after host-table.ss (ref-put!),
+;; to see a jolt-array (backed by a Chez vector). Loaded after host-table.ss (ref-put!),
 ;; transients.ss, seq.ss (the dispatchers it chains).
 
 (define-record-type jolt-array (fields (mutable vec) kind) (nongenerative jolt-array-v1))